In most computers, the Serial Port (COM Port) and Parallel Port are used to communicate with the outside world. The Parallel Port is also used to connect printers, and it is also known as the Line Printer Port (LPT Port). The Parallel Port, which transmits 8 bits of data at a time, is much faster than the Serial Port, which only transmits 1 bit at a time. However, the Parallel Port is generally used for short distance transmission because of the need of increasing data lines. Besides, the Line Printer Port can also be used to transmit data between two computers for short distance purposes.
Generally, there are 25 pins in a printer port. 17 of them (except 8 grounds, PIN 18˜PIN 25) are defined as follows:    1. Input: includes 5 STATUS signal pins, ACK (PIN 10), BUSY (PIN 11), PE (PIN 12), SLCT (PIN 13), and ERROR (PIN 15). They are used to input STATUS signals to computers.    2. Output: includes 4 CONTROL signal pins, STROBE (PIN 1), AUTOFD (PIN 14),INT (PIN 16). SCLTIN (PIN 17). They are used to output CONTROL signals from computers.    3. Input/Output: includes 8 data pins, D0 (PIN 2), D1 (PIN 3), D2 (PIN 4), D3 (PIN 5), D4 (PIN 6), D5 (PIN 7), D6 (PIN 8), D7 (PIN 9). They are used to receive and transmit data within computers.
With the traditional testing method there is no open circuit between input and output pins if the voltage level of the output signal of the output pin is equal to the voltage level of the input signal of the input pin. However, we can only test 10 out of 17 pins, the remaining pins are not testable. The traditional method also has the following disadvantages:    1. Short circuit between D5 (or D7) and the ground is undetectable.    2. Short circuit between any two pins between D0 and D7 is undetectable.    3. Short circuit between ACK and BUSY is undetectable.Therefore, testing departments for printer ports have a variety of solutions for short circuit detection. For example, traditional testing method has a particular circuit, as in FIG. 1. First, we check the voltage of the connected PIN 2 (D0) and PIN 15 by increasing or decreasing the voltage. If the voltage levels of the two pins are the same, the data will be transmitted from PIN 2 properly. Therefore, we can test all pins by connecting PIN 15 to D0˜D7. However, it is inefficient to test only one pin at a time, especially when we test the Extended Capabilities Port (ECP). Therefore, a solution to improve testing efficiency and simplify the testing procedure is necessary.